| Abstract: | SUMMARIES. An optical model was derived that predicted the spectral reflectance of a varnished oil painting based on the illumination angle of incidence, refractive indices of the varnish resin and paint-layer surface, spectral transmittance of the varnish, and internal spectral reflectance of the paint layer. A computational analysis was performed to predict common visual phenomena caused by varnishing, such as the increase in color gamut and contrast. Of particular interest was the importance of varnish resin refractive index on color appearance. The analysis revealed that the refractive index of the varnish resin has a very small impact on changing the appearance of a varnished painting, about one CIEDE2000 color-difference unit under exaggerated conditions. The model was also used to analyze how the addition of first-surface reflectance, caused by a varnish resin 'telegraphing' (replicating) the surface roughness of a paint layer, affects color appearance. In this case, large color changes occurred, about 20 CIEDE2000 color-difference units. Given that the ability of a varnish resin to level or replicate surface roughness is dependent on its molecular weight, it is clear that of the two physical parameters, refractive index and molecular weight, molecular weight is the dominant parameter in how a varnish resin affects the appearance of an oil painting.
CONCLUSION. The change in appearance of an oil painting following varnishing depends on properties of the paint layer, varnish, optical contact between the paint layer and varnish, and method of application. Paint-layer properties include surface roughness, porosity, amount of medium, RI at the surface of the painting, opacity, and internal spectral reflectance. Varnish properties include layer thickness, molecular weight (M W), surface roughness, RI, transparency, and internal spectral transmit-tance. (For a translucent varnish or paint layer, spectral absorption and scattering are required rather than internal spectral transmittance, or internal spectral reflect-ance, respectively.) The method of varnish application can change the surface roughness. Optical contact is an issue when it is not achieved, that is, if there is a layer of air between the varnish and paint. In this case, additional inter-reflections occur. Similarly, a varnish with trapped air-bubbles will scatter light; the air-bubbles behave in similar fashion to pigments whose RIs are different from those of their surrounding medium. An optical model was derived to predict the spectral reflectance of varnished and unvarnished paintings under the following assumptions. The paint layer was medium-rich, non-porous and opaque; the varnish was transparent and in optical contact with the paint layer; the MW of the varnish resin and the method of application yielded either complete leveling or complete replicating of the paint-surface roughness. The model parameters were the illumination angle of incidence, the RIs of the paint-layer surface and varnish, and the internal spectral reflectance of the paint layer. The model predicted common visual phenomena including the increase in color gamut and contrast when a new oil painting is varnished with natural varnish resins such as dammar and mastic or synthetic resins such as MS2A or Regalrez 1094. A computational analysis revealed that the RI of the varnish resin had a very small impact on changing the appearance of a varnished painting. Under the exaggerated conditions of 60° collimated illumination, the maximum C1EDE2000 color difference between a fresh linseed-oil black varnished with natural resin (RI = 1.54) or a synthetic resin with RI of 1.47 was 1.29. Typical museum lighting is much more diffuse and often of a smaller angle of incidence. Furthermore, resins with such a low RI are rarely used as varnish resins. Thus, differences in RI among typical varnish resins viewed under typical lighting conditions would result in even smaller differences in appearance. The optical model was also used to evaluate the color change caused by a varnish resin either completely leveling or replicating the surface roughness of a paint layer. For a fresh linseed-oil black, the maximum CIEDE2000 color difference between a resin that completely levels a diffuse surface and one that completely replicates the surface was 19.89. Thus, the changes in appearance caused by differences in surface roughness were more than an order of magnitude greater than those caused by differences in RI. The MW and RI of varnish resms have an inverse relationship, as shown in Table 1. With a decrease in resin MW, there is an increase in RI. Because MW has a strong influence on surface leveling, experiments concerned with the role of RI in the appearance of varnished paintings have been problematic, as pointed out in the introduction. This computational analysis clearly indicates that MW is the more important physical property of a varnish resin. This conclusion suggests that when considering new synthetic resins for picture varnishes, the key physical parameter with respect to appearance is molecular weight. |
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